Attempts have been made to produce a single stage to orbit (SSTO) vehicle. In order to be commercially viable, such a vehicle would generally require a high payload fraction in order that it may be adapted to meet different operational requirements. In addition, such a vehicle would be readily ground maneuverable and have a short maintenance turnaround cycle.
It is theoretically possible to realize an SSTO with high performance rocket propulsion. However, to use a rocket from take-off would necessitate a high payload of an oxidant, e.g. liquid oxygen, which would add considerable mass to the vehicle. One option is to augment the rocket engine with an alternative power propulsion unit and then to complete the ascent into orbit on rocket propulsion alone.
GB-A-2240815 describes a dual-mode or hybrid aerospace propulsion engine. In this engine, in a first mode of operation, the engine employs liquid hydrogen fuel to pre-cool the intake air of a turbo-compressor in order to deliver it, at high pressure, as oxidizer, to a rocket type combustor/nozzle assembly. At high Mach numbers, e.g. in excess of Mach 5, the engine changes to a second mode of operation which operates as a conventional high performance rocket engine using liquid oxygen carried on the vehicle to oxidize the liquid hydrogen fuel.
Such a hybrid engine can serve to extend the performance of the rocket engine by adding air breathing capability to it. Rocket engines are considered to be the most appropriate engine for achieving the necessary velocity to achieve orbit, for example, with an effective vacuum exhaust velocity (Vef) of about 4500 m/s.
In order that a common combustion and nozzle system can be employed in both propulsion modes (i.e. rocket and air-breathing modes), inlet air must typically be compressed to high pressure similar to, but not necessarily identical with, that in rocket operation (approximately 150 bar). In order to do this, the inlet air is first cooled, to keep the delivery temperature within practical limits (below 800K) and to minimize the compressor work required of the turbo-compressor.
However, such an engine can be difficult to control. The present disclosure seeks to alleviate, at least to a certain degree, the problems and/or address at least to a certain extent, the difficulties associated with the prior art.